1. Field of the Invention
This invention relates to ceramic compositions and their preparation. More particularly, it relates to diphasic xerogels and their use in preparing ceramic compositions. This invention especially relates to structurally diphasic xerogels and their use in preparing ceramic compositions
2. Background of the Invention
Innovative materials preparation, as recent history shows, has been the driving force behind a great deal of scientific technological innovation in materials science and engineering. The transistor-action discovery was clearly delayed until the germanium was purified sufficiently. From nylon to kevlar the polymer saga has been punctuated by the synthesis of new materials. In the ceramics age, diamond synthesis, the glass-ceramic process and product and ferrite and garnet compositional tailoring have all led to both new fields of science and of technology. In the modern ceramics processing of high-technology materials the need for ultrafine powders with controlled purity and homogenity has become paramount. These fine powders are often metastable and hence partake of many of the advantages (stored excess free energy) of such materials. Those working in this art are concerned with the intersection of two fields of preparation science: ultrafine powders and metasable solids.
Fine powders have been prepared by such prior art techniques as co-precipitation, co-decomposition and organic hydrolysis. During the late 1940's and early 1950's, the process known as sol gel process was developed where both alkoxide and SiO.sub.2 -sol precursors were employed for the preparation of fine noncrystalline ceramic and ultrahomogeneous glasses. R. Roy, J. Am. Ceram. Soc. 39,145 (1956). The sol gel process was employed to prepare literally hundreds of compositions in the common ceramic oxide systems involving Al.sub.2 O.sub.3, SiO.sub.2, MgO, TiO.sub.2, GeO.sub.2, ZrO.sub.2, UO.sub.2 and the like. In recent years, a few new glassy and ceramic materials have been added to the list of materials made by the sol-gel process. Recent research in this area has instead focused on the process itself insofar as it affected the reaction kinetics and microstructure of the gel-derived solid and on specific applications such as nuclear fuel pellets, coatings, fibers, and abrasive grains. Much recent work in single component systems has studied the effect of the gelling conditions on the crystallinity, microstructure, and porosity of oxides formed by sol-gel methods. Parameters such as the amount of H.sub.2 O, the acidity, and the gelation temperature have been found to be crucial in this respect. In the research to date, as far as can be determined, the universal goal has been to make a single phase xerogel solid. In other words, in the gel the solid is a single phase.
Heretofore gels have also been prepared in many multi-component systems, often with four or five components. However, these gels were single phase in their xerogel form, only upon annealing and crystallization do they typically yield several phases, both metastable and stable crystalline phases.
U.S. Pat. Nos. 3,979,215; 4,052,538 and 4,246,137 disclose the preparation of multicomponent ceramic articles where the preparation method includes the formation of a gel, which in all instances is a single phase gel, followed by a drying operation for production of the desired product.
U.S. Pat. No. 3,791,967 discloses a porous xerogel of alumina and/or silica containing hydrogenation metals and a tetravalent phosphate useful as a hydrodesulfurization catalyst. The preparation includes forming a hydrous gel containing preformed phosphate particles. The gel is recovered, washed and dried to provide the catalyst. There is no suggestion of the formation of a two phase gel in the procedure.
U.S. Pat. No. 4,314,827 discloses an aluminum oide-based abrasive mineral having a microcrystalline structure of randomly oriented crystal lites comprising a continuous phase of alumina and a secondary phase dispersed therein comprising (a) zirconia and/or hafnia, (b) a spinel derived from alumina and at least one oxide of cobalt, nickel, zinc or magnesium or (c) a combination of (a) and (b). These abrasive materials are prepared by forming a homogeneous mixture in a liquid medium of an alumina source compound, such as a colloidal dispersion or a hydrosol of alumina, and a precursor of the secondary phase, converting the mixture to a gel, drying the gel to obtain a porous solid material and firing the solid material under non-reducing conditions at a temperature of at least 1250.degree. C. to convert the solid material to the dense alumina-based abrasive mineral. No mention is made of the formation of other than a single phase gel.
The parent and grandparent patent applications of which this application is a continuation-in-part relate to the preparation of multiphasic xerogels using a sol-gel process which involves the preparation of inhomogeneous sols which are converted to multiphasic, especially diphasic xerogels. Diphasic xerogels of a ceramic oxide in combination with a metal, a metal compound or another ceramic oxide may be prepared by the procedures disclosed therein.
Thermal treatment is an essential operation in the preparation of ceramic oxide products. Typically, the thermal treatment at the lower temperature ranges of 0.degree.-400.degree. C. results in the removal of physically held water and organic binders and materials while thermal treatment in the upper temperature ranges, typically in the 700.degree.-1650.degree. C. range, provides both structural changes and compound formation. Thermal treatment in the upper ranges is often referred to in the art as "sintering." Structural changes include such changes as loss of crystallinity or the conversion from one crystalline form to another. For example, alumina crystalline structure changes with increasing sintering temperature from gamma to delta (ca. 850.degree. C.) to theta (ca. 1060.degree. C.) and finally to alpha-alumina (corundum) (ca. 1150.degree. C.). As the material is treated, a temperature is reached at which a new structural arrangement is more stable, the higher form usually having higher symmetry. In addition, compounds may be formed during high temperature thermal treatment. A typical example is the formation of spinel (MgAl.sub.2 O.sub.4) from its component oxides. Here, the reaction takes place in the absence of a liquid phase so that volume diffusion through the bulk material determines the reaction rate.
Improvements in the thermal treatment portion of ceramic oxide manufacture, particularly the sintering operation, are highly desirable. Significant reductions in the time or the temperature necessary to effect desired changes during sintering can substantially influence the economic attractiveness of any alterations in the manufacturing process.
It is an object of this invention to provide improvements in the process of preparing ceramic oxide compositions.
It is another object of this invention to prepare ceramic oxide compositions at lower sintering temperatures than employed heretofore.
It is a further object of this invention to prepare ceramic oxide compositions from structurally diphasic xerogels.